[student8607]

Today we started looking at molar masses and moles and etc
I'm a bit confused on the units and what means what
I picked out a few questions from the book to try to organize all this in my head. Also noted some questions.

A. If you have a dozen carbon atoms and a dozen oxygen molecules, how much do they weigh in amu?
-12x12.0g = 144.0g x (6.022x10^23 amu) / 1 g = 8.67x10^25amu
-12x(2x16.0) = 384.0g x (6.022x10^23 amu) / 1 g = 2.31x10^26amu

B. If you have a mole of carbon atoms and a mole of oxygen molecules, do you have the same number of carbon atoms and oxygen molecules?
No, because O is diatomic, so you would have 2x as many molecules as atoms.

C. How many Oxygen atoms do you have?
--?Where should I start - 1mol or 32g?
32g x (6.022 x 10^23 amu) = 1.93x10^25amu
--> is amu the unit for the number of atoms

D. Do the mole of carbon and the mole of oxygen have the same mass?
No, because they have different masses on the periodic table

E. How many molecules are in 5.78mol of propane?
5.78molC3H8 x 44g / 1 mol = 254g x (6.022x10^23amu) = 1.53x10^26amu
--> amu for # of molecules too?
--> I need to convert to grams first, right?

F. How many moles is 7.5x10^24 molecules of propane?
7.5x10^24molecules x (1.66x10^-24g) / 1amu = 12.45g
12.45g x 1mol / 44g = 0.28moles?
-->this many steps? the logic makes sense in my head. i just hope it's right on paper

G. Determine molar mass of KOH = 56.105g
-->my friend had g/mol but said he wasn't sure? it's not g/mol on the periodic table?

As always, thanks for your help. 

[azmanam]

amu and g are units of mass
molecules and moles are units of amount

amu is used when you're working on the order of individual molecules/atoms.  g is used when you're working on the order of moles of molecules/atoms.  It's just a matter of scale.

amu stands for atomic mass units.  Some perspective: the mass of one atom of carbon is ~1.99x10^-23 g. (really small number). To  manipulate numbers in those units is awkward and error prone.  Thus, we came up with the amu to simplify things.  1 amu = ~1.7x10^-24 g.  With this convenient unit, we can now say the mass of one atom of carbon is 12 amu.  It works out that amus and grams are related.  one carbon atom has a mass of 12 amu.  One mole of carbon atoms has a mass of 12 g.  Wikipedia does a pretty good job here.

http://en.wikipedia.org/wiki/Atomic_mass_unit

Now, as to your questions.  My first suggestion - especially because this is related to the topic of your post - always include units in your calculations.  And pay attention to your scale.  For your first question, you start your calculations with the number 12.  No good.  Gotta include units.  They're often given in the question.  12 carbon atoms.  This also tells us the scale we're working on - atoms.  One carbon atom has an atomic mass of 12.011 amu.  Use this conversion factor.

(12 carbon atoms)*(^{12.011 amu}/_{1 carbon atom}) = 144.132 amu.  Since we're already on the order of atoms, we don't have to divide by Avogadro's number here.  Same analysis for oxygen.

B - good.  but be careful with your wording.  O is not diatomic.  Molecular oxygen (O₂) is.

C - you should start with what is given in the problem: 1 mole.  amu is not the unit for amount.  atoms is the unit for amount.  amu is a unit of mass.

E - molar mass is not needed here.  What is the relationship between # of molecules and moles?

F - again, mass is not needed.

G - molar mass of a compound is the sum of the molar mass of the individual atoms.  The oxygen molecule has a molar mass of 32 because the sum of the molar mass of the individual atoms = 16 + 16 = 32.

[student8607]

Things are really starting to clear up now.

But I am still a little confused with

A. How much do the carbon and oxygen weigh in amu
If 1 amu = ~1.7x10-24 g
then how can
144g (12.0g for each of the 12 atoms) = 144amu?
thats 1:1?

[azmanam]

one atom (scale of atoms) of carbon does not have a mass of 12 g (scale of moles).

one atom (scale of atoms) of carbon has a mass of 12 amu (scale of atoms).

The question asked for the mass of "a dozed carbon atoms" in amu.  Start on one scale; stay on one scale.

The only time you could have atoms and grams in the same calculation would be to first convert from the scale of atoms to the scale of moles (or vice versa) by using Avogadro's number (6.23x10²³ atoms per mole).

[student8607]

ok, so AMU is used for the weight of atoms
--> is this true for molecules too?

so grams is used for the weight of what then?

[student8607]

C. How many Oxygen atoms do you have?
1mol x 32g/1mol = 32g x (6.022x10^23 atoms) = 1.93x10^25atoms
--> how does this look
--> still confused on the unit for the "6.022x10^23" it can be atoms OR amu?

E. How many molecules are in 5.78mol of propane?
--> molecules and moles both are for amount so are they equal? or is there a conversion factor? (6.022x10^23 ?)- it can be molecules too?

F. How many moles is 7.5x10^24 molecules of propane?
--> again: molecules and moles both are for amount so are they equal? or is there a conversion factor? (6.022x10^23 ?)- it can be molecules too?

[azmanam]

is this true for molecules too?

yes

so grams is used for the weight of what then?

g is used when you're working on the order of moles of molecules/atoms.

C - Avogadro's number is not a unit of mass.  It is a unit of amount.  The units of Avogadro's number is atoms/mole.  Or molecules/mole.  For questions of amount ("how many"), mass will not figure into the calculations.  for questions of mass ("how much"), then you'll need mass.

E - see above for units of Avogadro's Number.

F - same.

[student8607]

For questions of amount ("how many"), mass will not figure into the calculations.  for questions of mass ("how much"), then you'll need mass.

So, for "how many oxygen atoms do you have if you have 1 mole of oxygen molecules" mass would not figure it?
It would just be 1mol x avagrados number = avagrados number?
OR would you do ...
1mol O2 x 32g / 1 mole = 32g x 6.02x10^23 atoms = 1.93x10^25 atoms

For questions of amount ("how many"), mass will not figure into the calculations.  for questions of mass ("how much"), then you'll need mass.

So, for "How many moles is 7.5x10^24 of propane?"
would you just use avagrados number
OR would you do...
7.5x10^24 molecules x (1.66x10^-24g) / 1 molecule = 12.45g
12.45g x 1 mole / 44 grams = 0.28 moles C3H8


**last questions: I promise 

[azmanam]

Let's go through this stepwise.  Question asks for amount of oxygen in atoms (scale of atoms).  Question gives amount of oxygen molecules in moles (scale of moles).  This tells us several things immediately.  It's a 'how many' question, so answer will be in either atoms, molecules, or moles (at least at this point in the semester).  Also, because it's a 'how many' question, we won't need molar mass - in fact, we won't need mass at all (for now).  We start on the scale of moles and end on the scale of atoms, so we'll need to use Avogadro's number as a conversion factor.

ok, lets lay out what we know.  We know we have 1 mole of oxygen molecules.  We know 1 mole of something equals 6.02x10²³ molecules or atoms.  We know molecular oxygen is diatomic; thus, there are two atoms of oxygen in one molecule.

So let's put it all together.

1 mole O₂  | 6.02x10²³ molecules O₂  |    2 O atoms   |
       1      |            1 mole O₂          | 1 molecule O₂  |

1.20x10²⁴ atoms of oxygen in 1 mole of molecular oxygen.

Can you use the same analysis for the propane question?

[student8607]

Can you use the same analysis for the propane question?

5.78mol C3H8 x 6.02x10^23 molecules / 1mol = 3.48x10^34 molecules

Thanks so much for your help and patience.
Copied those steps into my notebook. They really helped.

[azmanam]

I didn't check your math, but setup is good.  Good job.

A suggestion:  when you get into more complex examples where (for example) you have to calculate amount of oxygen when you're given the amount of propane, you'll have to convert from one compound to another.  You do this by converting from moles of one compound to moles of another compound (don't worry about how right now, you'll get there).  Anyway, my suggestion is when you use the unit moles in your calculations, always indicate moles of what.  I would modify your setup like this:

5.78mol C3H8 x 6.02x10^23 molecules / 1mol C₃H₈ = 3.48x10^34 molecules C₃H₈

Getting into this habit now, will help avoid confusion later on.

[student8607]

will do.
thanks.